It is known to use discrete light emitting diodes (LEDs) for signalling and for graphic displays. LEDs have been available in red and green for some time, and , more recently, blue LEDs have become available; combinations of these colours have the ability to produce all colours, including white. U.S. Pat. Specification No. 4,992,704 describes the combination of red, green and blue LEDs in a unitary housing, of transparent or translucent material, to support red, green and blue LEDs in fixed, spaced relationship to each other. The housing is domed to form a lens; the purpose of the lighting device is to enable a signalling device or lamp to produce all colours in the visible light spectrum. Thus the signalling device or lamp can exhibit a change in colour, signalling a change in state.
It has now been found that a combination of red, green and blue LEDs can be used in connection with video screens, to produce a screen which has properties far superior to those presently experienced.
In various situations, it is desirable to use large screens of approximately billboard size. A major use of such screens is in shopping centres and malls, where large video screens can be use for point of sale advertising or entertainment.
Existing large video screen suffer from a major drawback. All displays on such screens include vertical and horizontal black lines. These arise because of the technology involved in translating the display to the large screen. Prior art screens are based on conventional cathode ray tubes, or use a rear projection three gun system, In every case, the resultant display on the large screen contains gaps, which appear as vertical and horizontal black lines, giving the display a "pasted" effect.
It is an object of the present invention, in one embodiment thereof, to provide a large screen which is capable of a video display in which these vertical and horizontal black lines cannot be detected visually.
Prior art screens also have the disadvantage of being quite bulky, For example, in the case of rear projection screens, any sized viewing area typically will be about two metres thick.
It is an object of the present invention, in an embodiment thereof, to provide a large screen which is substantially slimmer than these prior art screens.
Large video screens in current use also have other problems which make it difficult to use them in situations where normal/high ambient light conditions, wide viewing angle and short viewing distance prevail, for example, in shopping malls. Prior art large screen video displays suffer from one or more of the following problems: large grain (pixel) size, low light output and excessive directionality. As a consequence, the video images on these large screens often appear faded or dim and in addition cannot be seen clearly at oblique angles or at short viewing distances.
It is therefore another object of this invention to provide large screen video displays with high light output (brightness), high resolution and a wide viewing angle. It is a further object of the present invention to provide a large screen video display capable of producing photo-realistic images in full colour, with imperceptible flicker under all practical viewing conditions or image content, and free of image degradation due to gaps, motion smearing and directionality caused by physical limitations of the display medium.
In relation to large video screens, there is a problem in delivering power. Prior art methods of delivering power rely on the use of multi-stranded low resistance cable. In such a configuration, it is very difficult to minimise the loop area formed by the load, the power supply and the power supply leads. This results in lead inductance and transient oscillation on the power supply by the fast switching of
It is a further object of this invention in one embodiment thereof, to overcome the problems faced by the prior art methods and to deliver power to large video screen with a minimisation of direct current voltage drop and amplitude of the transient oscillation induced on the power supply by switching of the LEDs.